Structure–Reactivity Relationships in Carbon Nitride/Multi-Walled Carbon Nanotube Composites: UV–Visible Photocatalytic Performance

Introduction:
Hybrid photocatalysts based on polymeric carbon nitride (CNu) and purified multi-walled carbon nanotubes (pCNTs) have attracted attention due to their improved charge separation and photocatalytic efficiency. This study investigates the influence of pCNT incorporation on the physicochemical, photoelectrochemical, and photocatalytic properties of CNu-based nanohybrids.

Methods:
CNu was synthesized from urea in the presence of pCNTs by an in situ hydrothermal method. Different pCNT loadings (x, in mg) relative to 5 g of urea were used to obtain CNu/pCNT(x) composites. The materials were characterized using structural, morphological, surface, and photoelectrochemical techniques. Photocatalytic activity was evaluated through methyl orange (MO) degradation under UV and simulated solar irradiation.

Results:
pCNT incorporation induced significant structural and textural changes. ATR-FTIR analysis revealed an increase in surface –NH₂ groups with increasing pCNT content. XRD showed that CNu/pCNT(0.2) exhibited higher crystallinity than pristine CNu, whereas higher pCNT loadings reduced structural order. The composites displayed enhanced specific surface areas and micropore volumes, reaching 102.3 m² g⁻¹ for CNu/pCNT(2). The band-gap energy remained nearly unchanged (~2.9 eV). Photoelectrochemical measurements revealed increased photocurrent generation, indicating improved charge separation and transport. Oxygen reduction reaction studies under illumination showed enhanced activity, with CNu/pCNT(0.2) exhibiting the best performance. All samples showed enhanced MO removal under UV and simulated solar irradiation. For CNu/pCNT(0.2), the apparent photonic efficiency reached 21.3% under UV irradiation, 2.8 times higher than that of pristine CNu, indicating reduced electron–hole recombination.

Conclusions:
CNu/pCNT(0.2) showed the best photocatalytic and ORR performance. Its enhanced behavior is attributed to an optimal combination of crystallinity, textural properties, and charge-transfer efficiency, highlighting the potential of CNu/pCNT hybrids for environmental remediation applications.